Inductive power transfer, although contactless, has in most applications in the prior art using primary pathways required that the configurations shall include ferromagnetic cores and that the secondary or pickup shall be quite closely placed in proximity to, or about the primary conductor. For example, Kelley in U.S. Pat. No. 4,833,337 uses elongated ferrite inverted "U" cores and a ferrite member fixed to the primary pathway as well. BOYS& Green (WO92/17929) use "E" cores with one primary conductor located inside each space between the three limbs of the "E". Bolger (U.S. Pat. No. 3,914,562) teaches a 120 Hz primary inductive cable along a roadway, the cable having iron laminations along its entire length. These laminations face corresponding laminations within the moving vehicles that draw power from the tracks. These are expensive, heavy constructions which will exhibit magnetic attraction forces and any magnetostrictive effects within the cores will tend to cause noise. For transferring power to moving road vehicles, avoidance of core structures (at least in the primary pathway) and a wider tolerance in positioning is clearly useful.
Inductive power transfer systems in which various portions of the system are tuned to resonance are somewhat liable to instability should one or more resonant circuits assume a different resonant frequency to that of the system mean. Means to enhance stability are always useful, given that resonance is in most cases the preferred way to optimise the transfer of inductive power.
There are many applications in attention-gathering fields (i.e. advertising) in which it will be useful to extend the gap over which a useful field can be transmitted under inductive power transfer principles. Advantages of doing this include the concealment of the power sources so that panels appear to magically light up without a visible connection. Hence the use of inductive power transfer, which itself may involve higher frequencies, as a way of driving electroluminescent panels across a gap and without bare wires or contacts is a useful venture.
Electroluminescent panels have been available since at least 1957 as a source of lighting or of display and advertising material, yet they have proven to be difficult to drive at an acceptable level of brightness and at the same time retain a reasonably long life. Panels require a relatively high frequency (well above mains frequency) in order to glow at a useful level. Prior-art driving circuits such as dedicated chips rely on inverters to develop AC power at typically 800-1200 Hz, and up to typically 50 V peak-to-peak. Because the output of those inverters is substantially a square-wave waveform the phosphors of the panels are not excited optimally and brightness is not remarkable. Attempts to get more light with higher driving voltage usually results in breakdown of the dielectric and a failure of the panel, or a markedly curtailed life. There may be thermal runaway effects involved.